1. Technical Field
The present invention relates to a ceramic material for the encapsulation of high level radioactive waste, e.g. resulting from the reprocessing of irradiated nuclear fuel. The term reprocessing used herein includes not only processing which separates irradiated fuel to provide new fuel products but any processing which includes any separation of irradiated fuel, e.g. any so-called spent fuel reconditioning process.
2. Related Art
Vitrification has been the preferred method of encapsulating highly active wastes comprising fission products resulting from the reprocessing of irradiated fuels. The method involves the incorporation of the waste within a continuous amorphous matrix. However, waste streams which are likely to arise in the future due to developments to the so-called PUREX process (so-called Advanced PUREX process) may not be suitable for containment by the vitrification technique due principally to relatively high levels of iron, chromium and zirconium which result from the non-fuel components of fuel assemblies which are also taken into solution in the envisaged new reprocessing techniques. In particular it is envisaged that an electrochemical dissolution may be used at the Head End of a future. Advanced Purex reprocessing plant. In contrast to current shear/leech procedures, whole fuel assemblies will be dissolved continuously in an electrochemical cell. Approximately 15% of the Zircaloy cladding, together with all of the stainless steel and Inconel components of the fuel assembly are expected to be taken into solution and remain in the HA raffinate, after solvent extraction, in addition to the fission products and non-recycled actinides.
In addition to the need to reduce the costs of reprocessing operations, a further important consideration is that of minimising the quantity of waste produced without compromise to the durability of the waste form. In the context of developments to the PUREX process referred to above, this waste minimisation objective can be applied to comparisons with the total amount of waste produced by current PUREX reprocessing technology and also direct disposal. In conventional oxide fuel PUREX reprocessing the high level waste consists predominantly of fission products and is vitrified at a waste loading of 20–25 wt %. The high level waste produced by more modern and improved Advanced PUREX reprocessing routes, however, contains such high quantities of inert material from the fuel assembly that vitrification at the same waste loading would roughly quadruple the volume of high level waste produced per tonne of fuel reprocessed.
The waste loading is calculated as the mass of waste/total mass of waste immobilising medium or mass of waste/(mass of waste+mass of additives).
It is therefore desirable to be able to accommodate even higher loadings of active waste into the immobilising medium so as to minimise the volume of the final immobilised waste.
It would also be desirable to be able to provide improved operational stability of the waste immobilisation plant in that for a range of fuel assembly compositions the same composition and/or quantity of precursor for forming the immobilising medium may be used.
It is known to use a form of encapsulation utilising a ceramic having a range of crystalline phases wherein the waste ions are dissolved and held in solid solution in the ceramic matrix. An example of this latter form of encapsulation is the material known as “Synroc” (trade name) developed in Australia and comprises a matrix which includes zirconolite (CaZrTi2O7), perovskite (CaTiO3) and hollandite (BaAl2Ti6O16) as disclosed in European Patent Application No. 0007236.
However, it is considered that the conventional “Synroc” formulation may have disadvantages for encapsulating the waste envisaged from reprocessed fuel shortly to occur. In particular, because “Synroc” typically comprises a waste loading of only about 20 weight % and generally less than 30 weight %, the large amount of inert material produced with the new reprocessing methods will result in a large increase in the volume of the final immobilised waste.